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Thursday, 19 December 2019

Armature Reaction in DC Generator

When the generator is under operation, the current in the armature due to the e.m.f. generated produces a flux in the armature. The effect of the armature flux produced by the armature current distorts and weakens the main flux produced by the field poles due to which sparking at brush and low voltage at terminals may result. The study of the effect on the redistribution of flux under the main pole is done under armature reaction topic.

Whenever a brush contacts two or more commutator segments, the coils connected to these segments are short circuited for a brief period. After the period of short circuiting the current in these coils change their direction. The changes that may takes place in the coil after the period of short circuiting commutation. If the change is gradual it is called commutation and if it is sudden it is called rough commutation. Rough commutation leads to sparking at the brush contacts.

 To study the armature reaction the following terms are to be understood clearly.

a) Geometrical neutral axis (GNAT): 

lt is an axis midway between the opposite and adjacent poles. Geometrical neutral axis for two pole and four pole generator (without flux) is shown in Figure.

b) Magnetic Neutral Axis: 

The magnetic neutral axis is perpendicular to the lines of force between two adjacent opposite poles. 

(a) magnetic neutral axis due to field flux alone.
(b) the magnetic neutral axis due to armature flux alone.

c) Leading Pole Tip (LPT): 

When the armature rotates, the end of the pole through which the conductors enters the magnetic region is called the Leading Pole Tip (LPT).

d) Trailing Pole lip (TPT): 

It is also sometimes called Lagging Pole tip. It is the tip of the pole through which the conductor leaves the magnetic region. H. the direction of rotation is changed the leading and trailing tips also change. Refer to Fig.

To study the effect of armature flux on the field flux, it is necessary to consider separately the distribution of magnetomotive force of the main field and armature. For the purpose of the convenience a bipolar generator is taken to illustrate the effect of armature reaction.

1. Distribution of Main Field E.m.f:

Consider a bipolar generator with armature at the centre kept stationary and the main field excited. Since there is no rate of change of flux linkage in the armature, the e.m.f. induced in the armature conductor is zero. Hence, the distribution of flux'due to main pole is only shown. In this condition the distribution of flux in the air gap is uniform and the magnetic neutral axis (MNA) and geometrical neutral axis (GNA) coincide. This condition can also occur where a generator is on no load.

2. Distribution of Armature E.m.f.

Consider the flux produced due to armature current alone. In this case; the distribution of flux is as shown in Fig. The direction of armature current would be the same as it would actually be when the generator is on load. Using the Flemings right hand rule the direction of induced e.m.f. is found.
The downward current is represented by a cross and upward current is represented by a dot. The net effect of the magnetic flux around individual conductors is to send the flux downwards through the armature. This can be found out by using cork screw rule. The magnitude of armature e.m.f. depends upon the magnitude of armature current. In this case also the Geometrical Neutral Axis (GNA) and Magnetic Neutral Axis (MNA) coincide.

3. Distribution of Resultant E.m.f. :

The distribution of flux due to main pole is to-wards south pole. In Fig (a) under north pole Φp represents the direction of magnetic flux in the north pole, Φa represents the direction of magnetic flux in the north pole due to the armature current. The resultant of Φp and Φa is ΦR whose direction is towards the trailing pole tip (TPT). Therefore, the flux inside the north pole- would bend towards TPT which causes crowding of flux at TPT and wearing of flux at leading end of north pole.
In Fig. (a) in the armature, the direction of flux of the mainpole (Φp) is again towards south pole but the direction of flux due to armature current (Φa) is downward towards the geometrical neutral axis. The resultant direction of (Φp), and (Φa) is (ΦR) which takes a shift from the axis of the pole. The angle of shift is θ°. The direction of flux due to the main pole Φp, in south pole coincides with pole axis and is shown in Fig. (a) and the direction of flux (Φa) due to armature current in the south pole is upwards towards the geometrical neutral axis. The result of Φp and Φa is ΦR: The flux bends again and resumes its normal pole axis. Thus, there is a shift in the direction of flux from the normal pole axis to an axis with a shift of θ°. The complete resultant flux pattern due to the effect of armature flux on the main pole flux is shown in Fig (b).

Before considering this reaction the magnetic neutral axis was in line with the geometrical neutral axis. The magnetic neutral axis (MNA) shifts by an angle θ perpendicular to the resultant mmf due to which the brush takes a forward lead through an angle θo to lie in its new position of MNA. Due to this shift the armature current gets redistributed and some armature conductors which were earlier under the influence of north pole come under the influence of south pole and vice versa. The angle θ° of shift of magnetic neutral axis (MNA) increases with the increase in armature current due to increase in load. If the brushes arc not shifted to this MNA sparking may occur brush contact points. .

4. Effect of Armature Reaction :

The following are the effects of armature current on the machine

a. It distorts uniformity of the main flux and hence has cross magnetising effect.
b. It produces a demagnetising effect on the main pole.
c. It reduces the e.m.f. induced in the armature.
d. if brushes are not shifted to MNA sparking at brush contacts occur.
e. In self excited generator, short circuit creates heavy armature reaction resulting in demagnetisation of pole cores. The result is that the residual magnetism is completely lost.
f. The efficiency of the generator decreases.

5. Demagnetising and Cross Magnetising Conductors:

Due to the armature current, there is a shift of magnetic neutral axis (MNA) by an angle θo which is dependent on the load on the generator. Before the shift of MNA the magnetic neutral axis and geometrical neutral axis were same. The conductors which were adjacent to the geometrical neutral axis upto an angle of θo were outside the influence of the pole and no e.m.f. were induced in that conductor. Before the shift of MNA, the effect of these conductors, is to distort the main field. Now consider the effect of these conductors which are still adjacent to geometrical neutral axis even after the shift of MNA. The conductors lying upto an angle θo on both sides of geometrical neutral axis at top and bottom are subjected to a flow of current due to the induced e.m.f. in the rest of the armature conductors in such a direction as to set up magnetic flux around it in the armature. The direction of magnetic field of these conductors is opposite to the direction of the magnetic field set up by the conductors adjacent to the GNA at top and bottom upto an angle of θo is to demagnetise the main flux.

Now consider the conductors on both sides of the MNA and under the pole influence. It sets up a current in such a direction that the magnetic flux produced by it in the armature will distort the main flux giving an effect of cross magnetisation. These conductors are known as cross magnetising conductors. Both these demagnetising and cross magnetising conductors play an important role in the armature reaction.

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